1. Field of the Invention
The present invention relates to a photothyristor device, a bidirectional photothyristor device and an electronic apparatus having high breakdown voltage. More particularly, the invention relates to a photothyristor device, a bidirectional photothyristor device and an electronic apparatus for use with a light-triggered SSR (Solid State Relay).
2. Description of the Background Art
Conventionally, there has been known a field plate structure in a high breakdown voltage photothyristor device as shown in FIGS. 7 and 8. Referring to FIGS. 7 and 8, an n-type diffusion region 102 serving as a channel stopper is formed on the perimeter of the surface of an n-type silicon substrate 101. Inside n-type diffusion region 102, an anode diffusion region 103 (103′), a p-type gate diffusion region 104 (104′), a gate resistance diffusion region 105 (105′), and a cathode diffusion region 106 (106′) are formed by selective diffusion. Oxide films 110 are formed as insulating films covering the region of n-type silicon substrate 101 ranging from n-type diffusion region 102 to anode diffusion region 103 (103′), and the region of n-type silicon substrate 101 ranging from anode diffusion region 103 (103′) to p-type gate diffusion region 104′ (104) and gate resistance diffusion region 105 (105′). On oxide film 110 on the region of n-type silicon substrate 101 from n-type diffusion region 102 to anode diffusion region 103 (103′), a semi-insulating oxygen-doped polysilicon film 111 is placed. A silicon nitride film 112 is placed thereon.
In a ch1 (channel 1) of the two photothyristor devices, a T1 electrode 107 and a channel stopper electrode 108 are in ohmic contact with anode diffusion region 103 and n-type diffusion region 102, respectively. Also, in a ch2 (channel 2), a T2 electrode 107′ and channel stopper electrode 108 are in ohmic contact with anode diffusion region 103′ and n-type diffusion region 102, respectively. When ch1 is operated and a positive bias is applied to T1 electrode 107, a negative (−) potential is applied to p-type diffusion region 104 through T2 electrode 107′ for reverse-biasing, and a positive (+) potential is applied to n-type diffusion region 102 through channel stopper electrode 108.
Due to the voltage relationship as described above, a small current is generated in oxygen-doped polysilicon film 111 between T2 electrode 107′ and channel stopper electrode 108. Fixed charge is generated in the field area at the surface of the device by oxygen-doped polysilicon film 111. Further, a depletion layer is generated from p-type gate diffusion regions 104 toward the silicon substrate. Here, it is necessary to control the small current in oxygen-doped polysilicon film 111 to generate desired fixed charge. This may cause a leak current in the device; therefore, the film quality must be optimized.
The voltage relationship described above alleviates concentration of the electrical field within the device, thereby improving the breakdown strength of the device. In FIGS. 7 and 8, T1 electrode 107 and T2 electrode 107′ are extended toward the channel stopper electrode beyond the pn junctions at the n-type silicon substrate 101 surface, in order to facilitate the field plate effect of T1 electrode 107 and T2 electrode 107′. Namely, if such a structure are not employed, depending on the resistivity of oxygen-doped polysilicon film 111, a positive fixed potential, called Qss, and positive charge of natrium or the like within oxygen-doped polysilicon film 111 would cause the surface of n-type substrate 101 to be further n-typed, making it difficult to expand a depletion layer from p-type anode diffusion region 103 (103′) toward n-type silicon substrate 101. This would result in a breakdown in the vicinity of the pn junction interface at the surface of anode diffusion region 103 (103 ′). In order to prevent this, electrodes 107 and 107′ are extended to form a so-called overlay structure.
Next, an example of another conventional high breakdown strength photothyristor will be described with reference to FIGS. 9 and 10 (see Japanese Patent Laying-Open Nos. 08-130324 and 2002-190613). In the example of the prior art shown in FIGS. 9 and 10, a portion 13, which is adjacent to electrode 107 (107′), of oxygen-doped polysilicon film 111 for generating a fixed-potential in the field area is selectively doped with impurities such as phosphate or boron to make a low-resistant portion at this portion. Electrode 107 (107′) is placed not to be overlaid on the upper portion of the pn junction interface between anode diffusion region 103 (103′) and p-type gate diffusion region 104 (104′) and the silicon substrate, at the surface of n-type silicon substrate 101, and low-resistance portion 113 which is optically transparent is used as a field plate electrode.
With such a structure, the light-receiving area is increased without affecting the breakdown strength; therefore, a high sensitivity light-receiving device may be realized.
Photoelectric devices such as photodiodes, phototransistors and photothyristors are required to have high light-sensitivity for incident light and also required to convert light signals into electrical signals with the smallest possible chip size. However, in order to realize a high breakdown voltage characteristic, in the case of the photothyristor shown in FIG. 8, a field plate electrode of Aluminum (Al) etc., must be overlaid on the pn junctions having a high sensitivity. This results in significant reduction of the light sensitivity.
Also, with the method which dopes the high resistance film with impurities for improving the light sensitivity as disclosed in Japanese Patent Laying-Open Nos. 08-130324 and 2002-190613, Qss is increased due to the impurity doping and an unnecessary level is generated at the n-type silicon substrate surface. This causes, in the case of a photothyristor, reduction or variations in the current amplification factor hFE of the lateral type pnp transistor. This leads to variations of the light sensitivity.